1. Technical Field
The present invention relates to a liquid crystal display device and a method for the manufacture thereof, and more specifically, to a projection-type liquid crystal display device using liquid crystal light valves, and a method for the manufacture thereof.
2. Description of the Prior Art
In recent years, a projection type liquid crystal display device has assumed a new prominence as a potential ultra-high definition display device which replaces CRTs. The projection type liquid crystal display device has already been used in HDTV or OEP displays.
The projection optical system of the projection type liquid crystal display device consists of a light source, light valves, a screen, optical filters, and projection lenses. Liquid crystal panels are used as the light valves. Light valves are classified into a transmission type liquid crystal light valve, which transmits light from the light source and projects images on the screen, and a reflection type liquid crystal light valve, which reflects light from the light source and projects images on the screen. Since liquid crystal light valves generally reflect/transmit the lights of three primary colors, red (R). green (G). and blue (B), three light valves are used in a projection type liquid crystal display device.
A liquid crystal display device of the active matrix type consists generally of an array substrate on which there are interconnected switching elements and display electrodes and a counter substrate on which there are a counter electrode facing to the array substrate at a predetermined distance (cell gap) from the array substrate. Liquid crystal material is enclosed between the array substrate and the counter substrate.
In order to achieve desired electro-optical properties of the liquid crystal, the predetermined cell gap must be maintained evenly throughout the entire display surface of the panel. To accomplish this, some display devices use a large number of glass or plastic beads having a diameter of several microns dispersed on the panel as spacers to make the cell gap even. However, this method with using spacers has problems of the uniformity of beads diameters and difficulty in the even dispersion of spacer beads on the panel, as well as the loss of light due to spacers on pixels.
Instead of the above spacer bead dispersion method, a method has been proposed of forming columns consisting of an insulation film and the like in the cell gap as spacers. In this method, columns of a silicon oxide film are formed in the cell gap as spacers (pillar-shape spacers). by using photolithography commonly used in the manufacturing process of semiconductor devices. This method is advantageous compared with the conventional method of using spacer beads in that a cell gap is formed with a high accuracy.
For example, in some reflection type liquid crystal light valves where a silicon substrate is used as the array substrate, an MOS transistor as the switching element and an aluminum (Al) reflecting film connected to said MOS transistor are provided on the array substrate for each element, and a plurality of pillar-shape spacers are formed in the shaded on black matrix region between reflecting films.
In a projection type liquid crystal display device which displays high-definition images on a large screen, the improvement of display brightness is important.
In order to improve display brightness an increase in the numerical aperture of subpixels is considered. In a reflection type liquid crystal light valve, pillar-shape spacers are formed between two reflecting films, and the ends of the spacers override the reflecting films to decrease the reflection area of the reflecting films, causing the numerical aperture of subpixels to lower.
Also in the cell gap forming system using such pillar-shape spacers, even if the overriding of spacers on the reflecting films as described above could be prevented not to affect the numerical aperture, other factors lowering display qualities still remain.
First, where there are pillar-shape spacers formed between reflecting films, the application of an oriented film is affected by projecting pillar-shape spacers. The uneven application of the oriented film makes it difficult to obtain a cell gap which is even throughout the surface, and may cause the cell gap to vary. The regions of varied cell gap do not provide same contrast obtained from the proper cell gap.
Second, reverse tilt occurs in liquid crystals present in the close vicinity of the of the spacers cause reverse tilt due to the pillar-shape spacers. As a result of this disturbance of orientation a discontinuity is observed as a line, known as a disclination line, occurs causing contrast to lower, and residual images to occur.
In a projection type liquid crystal display device, where three light valves are used for three primary colors, red (R), green (G) and blue (B) to display enlarged images on a large screen, decreases in numerical aperture, and contrast due to the variation of the cell gap, and the occurrence of disclination lines are particularly noticable.
The disturbance of liquid crystal orientation of pixels cannot be avoided as long as pillar-shape spacers are present on the pixel region contributing to the display. In high-definition, large-screen projectors, the disturbance of liquid crystal orientation is enlarged, and projected on the screen as the screen becomes larger, causing the increased degradation of the display quality.
It is an object of the present invention to provide improved display quality in a liquid crystal display device comprising pillar-shape spacers formed in the pixel array region by preventing contrast from degrading and residual images from occurring. Another object of the invention is to provide a method for the manufacture of such a liquid crystal display device.